lt3694-1 Linear Technology Corporation, lt3694-1 Datasheet - Page 15

• Manufacturer Part Number: lt3694-1
• Description: Lt3694/lt3694-1 - 36v, 2.6a Monolithic Buck Regulator With Dual Ldo
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The power section, consisting of the modulator, power
switch and inductor, is modeled as a transconductance
amplifier generating an output current proportional to
the voltage at the V
integrates this current, and that the capacitor on the V
pin (C
resulting in two poles in the loop. In most cases a zero
is required and comes from either the output capacitor
ESR or from a resistor R
model works well as long as the value of the inductor is
not too high and the loop crossover frequency is much
lower than the switching frequency. A phase lead capaci-
tor (C
transient response.
Figure 3 shows the transient response when the load
current steps from 1A to 2.6A and back to 1A.
BST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram in Figure 1) are used to generate a boost
voltage that is higher than the input voltage. In most cases
a 0.22μF capacitor will work well. Figure 4 shows three
ways to arrange the boost circuit. The BST pin must be
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
100mV/DIV
Figure 3. Transient Load Response of the LT3694
Front Page Application as the Load Current Is
Stepped from 1A to 2.6A. V
PL
C
1A/DIV
) integrates the error amplifier output current,
) across the feedback divider may improve the
V
OUT
I
L
C1
pin. Note that the output capacitor
100μs/DIV
C
in series with C
OUT
= 3.3V
36941 F03
C
. This simple
C1
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BST pin
operation with 2.5V outputs, use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (see Figure 4b). For lower output voltages, the
BIAS pin can be tied to the input (Figure 4c), or to another
supply greater than 2.8V. Tying BIAS to V
maximum input voltage to 7V. The circuit in Figure 4a is
more efficient because the BST pin current and BIAS pin
quiescent current comes from a lower voltage source.
One must also ensure that the maximum voltage ratings
of the BST and BIAS pins are not exceeded. The minimum
Figure 4. Three Circuits for Generating the Boost Voltage
4.7μF
4.7μF
4.7μF
V
V
V
IN
IN
IN
(4c) For V
(4b) For 2.5V < V
V
V
V
IN
IN
IN
(4a) For V
LT3694
LT3694
LT3694
BIAS
BIAS
BIAS
GND
GND
GND
OUT
LT3694/LT3694-1
< 2.5V; V
BST
BST
BST
SW
SW
SW
OUT
OUT
> 2.8V
IN(MAX)
C3
C3
< 2.8V
C3
D2
= 7V
36941 FO4
IN
V
OUT
reduces the
V
V
OUT
OUT
15
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